This invention relates to a process for the production of isotactic polypropylene and the crystalline copolymers thereof with up to 10 molar percent of butene-1 employing a mixed catalyst.
The low-pressure polymerization of propylene is generally conducted in hydrocarbons which are liquid under normal conditions, e.g., hexane, heptane or a higher-boiling aliphatic hydrocarbon. This has the disadvantage that an evaporative cooling can only be employed under low pressure so that the drying step is relatively expensive. Therefore, it is more advantageous to effect the polymerization of propylene in the monomer and/or in propylenepropane mixtures. However, due to the high vapor pressure of propylene, high pressures are required for this purpose, for example in case of polymerization temperatures of 70.degree.-80.degree. C., pressures of 31-37 atmospheres gauge must be employed. In the presence of hydrogen for molecular weight control, correspondingly higher pressures result. However, reactors of a large volume which can be used at these pressures can no longer be economically manufactured.
Therefore, it would be desirable if a process were available which permits the polymerization of propylene at lower pressures than is possible with butane but which permits evaporative cooling.
With respect to vapor pressure, butane is a suitable diluent. However, it has the disadvantage that the polymerization in commercial, dried butane takes place only at a low space-time yield. Butene-1 is unsuitable as a diluent, since it is polymerized along with the propylene with Ziegler-Natta catalysts. According to U.S. Pat. No. 2,956,989, butene-2 is also unsuitable as a diluent, since it is polymerized in the presence of Ziegler-Natta catalysts. According to work carried out by R. O. Symcox (J. Polymer Sci., Part B, 2 [1964] No. 10, pp. 947-949), the product is not polybutene-2, as disclosed in U.S. Pat. No. 2,956,989, but is polybutene-1, since the butene-2 is isomerized by Ziegler-Natta catalysts to butene-1, which is then polymerized to polybutene-1. The work conducted by Symcox is confirmed by further publications, e.g., T. Otsu, J. Polymer Sci. A 4 (1966), No. 6, pp. 1579-1593; Masao Iwamoto and Sadao Yuguchi, Bull. Chem. Sre., Japan, 40, pp. 159-162 (1967); German Unexamined Laid-Open Application DOS No. 1,545,042; French Pat. No. 1,415,239. According to these publications, butene-2 is not feasible as a diluent for the propylene polymerizations employing Ziegler-Natta catalysts, since, in addition to the polypropylene, larger amounts of polybutene-1 and butene-propylene copolymers would be produced.